As it is well known, the production in the oil wells generates an oil flow mixed with water (either of injection for secondary recovery, or as emerging water) and gas (mainly methane). The presence of gas shall depend on the particular type of basin and well, but for the practical purposes of the neutron radiation, they represent an “empty” volume and do not contribute to the proceeding. The flow drags salts, mud and other suspended particles, that are not considered a “phase” in themselves due to their relatively low concentration.
The problem of a multiphase flow lies in that there are relative speeds between the phases, as well as a volumetric fraction for each of them, what makes it necessary to have information regarding some of these aspects to know the total mass and volumetric rate. In the oil production in particular, there exists a maximum “water cut” (volumetric fraction of water mixed with oil) to justify crude oil extraction, close to 98%. In these cases, the oil is transported as emulsion over the water phase and there are no relative speeds.
If the system presents a gaseous component, the complexity increases since this phase is compressible and the volumetric flow may vary hundreds of times between wells. The flow pattern in a mixture of gaseous and liquid phases shall also depend on the relative speeds between them as well as on the volumetric fraction that each of them occupies.
Consequently, in order to quantify the production in “wellhead” it is necessary to determine the amount of gas, water and oil that circulate, for the pressure and temperature conditions at which the station works. This allows extending the useful life of the well since it provides the producing company with the information necessary for the optimal management of the well.
This current state of the art allows measuring in a precise manner flows of single-phase systems, and also two-phase systems, using correction tables. However, these methods are inaccurate and highly dependent on the conditions in which these tables were obtained. For three-phase systems there are flowmeters based on Gamma radiation sources. Other technological alternatives to determine the volumetric fraction of the phases consist on ultra-sound, capacity impedance, among others.
The present invention solves above-mentioned problems using conventional measurements of pressure drop in restrictions to calculate the total flow and nuclear measurements. These last ones are based on placing a portable neutron source (of the Am—Be type sealed in stainless steel) in the proximity of a pipe section, the radiation interacts with the fluids and as a result a neutrons field of distinctive characteristics is generated, which can be quantified with a specific detection bench (gaseous detectors, of the proportional counter type).
The suggested technique is called “neutron interrogation” and allows measuring variations in the composition of each chemical element present in the mixture, with special sensitivity to the presence of “moderator” elements such as, mainly, carbon and hydrogen. Therefore, when the flow has variations in the water cuts as well as in the gas flow, the detected calculation of neutrons shall change in an approximately proportional manner.
Based on the calibration curves, for known pressure and temperature conditions and a defined geometry it is possible to contrast the measurements of the detectors to determine the volumetric fraction of each phase. The devise has a section where a “Venturi” restriction was implemented, which generates a pressure loss that accounts for the total volumetric flow that circulates trough the conduct. Consequently, the invention allows the measurement in an efficient and unambiguous manner of the total mass and volumetric flow, as well as of the partial corresponding to each constituent phase.
The patent documents U.S. Pat. No. 4,856,344 and 5,591,922 describe devises for measurement of multiphase flow speed of the Gradio-Venturi type which through measurements of differential pressures distributed along the device, provide information relative to the flow speeds of the phases. In both patents, the Bernoulli principle is used to associate the pressure drop (either in the throat of Venturi type section or pipes of constant section) to calculate both the volumetric fraction that each phase occupies as well as the total flow. To do so, pressure drop measurements adequately combined are used (Venturi consecutive sections or measurements of pressure drop due to friction) being this the inventive contribution in each case. As a difference with the filed invention, where the Venturi section is used as support based on the data obtained from the measurements of neutron radiation detectors; that is to say, once the vacuum and water cut of the mixture are known. Therefore, the utility given to the measurement in Venturi is equivalent to the conventional measurements in single-phase flows.
The patent documents U.S. Pat. No. 5,543,617 and 4,825,072 refer to a method for measurement of flow speeds using techniques that include radioactive tracers; the method includes injecting in the flow a non-radioactive tracer that has a transversal section for neutrons capture bigger than the one of the fluid which flow is desired to be measured, the introduction of the tracer that can be a Gadolinium composition is introduced upstream of a pulsed source that generates neutrons that radiates the fluid; the measurement of the Gamma rays induced by capture of neutrons, may be made with a scintillation detector and based on this measurement determine the correlation with the flow speed. The differences of said documents regarding the proposed device consist in the following three items: discrimination method, perturbation over the flow and radiation source used.
The discrimination of the phase is made when analyzing the moderation capacity of the multiphase mixture. When changing the relation of the phases a change in the pipe leakage spectrum is detected, that is proportional to the presence of gas, water and oil. The dependence with the phases' content is not only energetic but also directional. To take advantage of said effect there are neutron detectors calibrated in different range of energy in transmission and reflection positions regarding the issuing source.
On the other hand, as it dispenses of tracers, it is not necessary to consider the effect of the perturbations in the circulation pattern. In this manner the mixture of fluids is analyzed without modifications in its pattern.
The type of source used is different from the prior models. A fixed source is used (isotopic) of constant issuance instead of a pulsed issuer. In this manner the discrimination in energetic efficiency of the detectors (fast and thermal) and in efficiency due to angular position regarding the source is used to quantify the water cut and gas fraction in the mixture.
The European patent application EP 2551648 A1 refers to an apparatus for the measurement of a flow speed of a multiphase fluid sample that includes at least a gaseous phase and a liquid phase, where the apparatus includes a multiphase flowmeter; said device is formed by a pipe section through which the mixture of multiphase fluid enters to a measurement section that estimates a representative fraction of at least one of the gaseous phase and the liquid phase; a device of the mixture of multiphase fluid that passes through the measurement section; where the multiphase flowmeter further includes a reference measurement apparatus coupled to said multiphase flowmeter to provide in the place a measurement of the composition of an extracted sample, the flowmeter of this antecedent uses a Gamma source instead of the neutron source used by the present invention.
The physical principle used to quantify the volumetric fraction of the liquid and gaseous phases is different. In the present invention the phenomenon of “moderation” of fast neutrons within a media with low absorptions is used; being the main advantage the high degree of sensitivity to the presence of water and hydrocarbons. In invention EP 2551648 A1 the phenomenon of Gamma photons attenuation, in particular of an isotopic source of Bario-133, is used. As can be consulted in the open literature (Handbook of MultiphaseFlowMetering, Revision 2, March 2005, Norway. ISBN 82-91341-89-3), the source produces Gamma photons of two specific energies that are attenuated when passing through the conduct and the multiphase mixture. The corresponding detectors register the counting in the energies of interest and in this way the proportion between each present phase is calculated. The Gamma radiation is attenuated with better efficiency with higher atomic number of the nuclei of the present atoms: preferably with the steel of the conduct and structures and with lower probability in the nuclei of hydrogen and carbon that are intended to be detected. In this sense the neutron radiation presents an inherent advantage for these measurements, not being responsiveness to the surrounding structures but with high degree of response before changes in the amount of water, oil or gas.